Cross-sectional scanning of arteries is performed by sweeping an acoustical beam repeatedly though a series of radial positions within a well-defined cross-sectional plane. The acoustic beam is swept by either a mechanically rotated acoustic element or electronically switched elements. For each radial position, the echoes, which contain physical information about the surrounding area, are sampled with the resulting values stored as lines in a scan converter memory. Each line corresponds to the radial position of the acoustic beam at the moment the echoes were created. Within the scan converter, the sampled echoes will be integrated to form a cross-sectional image of the artery. Using continual imaging techniques, the cross-section of the blood vessel and the lesion is displayed on a TV monitor. This image on the monitor, correctly displays the intima, media, adventitia, plaque, and in some cases the structure of the lesion.
Sweeping the acoustic beam is accomplished by either rotating a transducer or rotating a mirror. If the transducer is rotated, the shaft must be very flexible as it contains all of the electrical wires for the transducer. When the mirror is rotated, the non-moving transducer avoids the necessity of rotating electrical wires. However in each of these methods, the focal length of the acoustic beam is set, which leaves the best resolution at a fixed radius. This fixed radius may not correspond to the region of interest in the blood vessel.
Finer resolution is needed at different radii to better characterize the nature of a lesion. Unfortunately, as the transducer and the mirror are fixed in space, the focal length is also fixed. There are no provisions for dynamically changing the effective radius along the arterial wall. Vital information, which could lead to better treatment, is lost or degraded.